1. Field of the Invention
The present invention relates generally to a method and apparatus for more accurately measuring the temperature of a surface using infrared measurement techniques and, more particularly, to such a method and apparatus which utilizes a laser sighting device which is adapted to project a circumscribing laser sighting beam or beams for more clearly defining the periphery of the energy zone from which the temperature is measured. Generally speaking, this accomplished by rotating the laser beam about the periphery of the energy zone; by the use of two or more stationary laser beams which are focused on the periphery of the energy zone; or by the use of a controlled single laser beam directed towards two or more predetermined locations on the periphery of the energy zone. In an alternative embodiment, a single laser beam may be rotated around the periphery of the energy zone using, for example, slip rings. In another embodiment, the single rotating laser may be pulsed on and off in a synchronized manner in order to produce a series of intermittent lines outlining the energy zone, thus increasing the efficiency of the laser by concentrating its total wattage in a smaller area, causing a brighter beam.
2. Description of the Prior Art
Remote infrared temperature measuring devices (commonly referred to as infrared pyrometers or radiometers) have been used for many years to measure the temperature of a surface from a remote location. Their principle of operation is well known. All surfaces at a temperature above absolute zero emit heat in the form of radiated energy. This radiated energy is created by molecular motion which produces electromagnetic waves. Thus, some of the energy in the material is radiated in straight lines away from the surface of the material. Many infrared radiometers use optical reflection and/or refraction principles to capture the radiated energy from a given surface. The infrared radiation is focused upon a detector, analyzed and, using well known techniques, the surface energy is collected, processed and the temperature is calculated and displayed on an appropriate display.
Examples of such infrared radiometers are illustrated at pages J-1 through J-42 of the Omega Engineering Handbook, Volume 2B. See, also, U.S. Pat. No. 4,417,822 which issued to Alexander Stein et al. on Nov. 29, 1983 for a Laser Radiometer; U.S. Pat. No. 4,527,896 which issued to Keikhosrow Irani et al. on Jul. 9, 1985 for an Infrared Transducer-Transmitter for Non-Contact Temperature Measurement; and U.S. Pat. No. 5,169,235 which issued to Hitoshi Tominaga et al. for Radiation Type Thermometer on Dec. 8, 1992. Also see Baker, Ryder and Baker, Volume II, Temperature Measurement in Engineering, Omega Press, 1975, Chapters 4 and 5.
When using such radiometers to measure surface temperature, the instrument is aimed at a target or "spot" within the energy zone on the surface on which the measurement is to be taken. The radiometer receives the emitted radiation through the optical system and is focused upon an infrared sensitive detector which generates a signal which is internally processed and converted into a temperature reading which is displayed.
The precise location of the energy zone on the surface as well as its size are extremely important to insure accuracy and reliability of the resultant measurement. It will be readily appreciated that the field of view of the optical systems of such radiometers is such that the diameter of the energy zone increases directly with the distance to the target. The typical energy zone of such radiometers is defined as where 90% of the energy focused upon the detector is found. Heretofore, there have been no means of accurately determining the size of the actual energy zone unless it is approximated by the use of a "distance to target table" or by actual physical measurement
Target size and distance are critical to the accuracy of most infrared thermometers. Every infrared instrument has a field of view (FOV), an angle of vision in which it will average all the temperatures which it sees. Field of view is described either by its angle or by a distance to size ratio (D:S). If the D:S=20:1, and if the distance to the object divided by the diameter of the object is exactly 20, then the object exactly fills the instruments's field of view. A D:S ratio of 60:1 equals a field of view of 1 degree.
Since most IR thermometers have fixed-focus optics, the minimum measurement spot occurs at the specified focal distance. Typically, if an instrument has fixed-focus optics with a 120:1 D:S ratio and a focal length of 60" the minimum spot (resolution) the instrument can achieve is 60 divided by 120, or 0.5" at a distance of 60" from the instrument. This is significant when the size of the object is close to the minimum spot the instrument can measure.
Most general-purpose IR thermometers use of focal distance of between 20" and 60" (50 and 150 cm); special close-focus instruments use a 0.5" to 12" focal distance. See page Z54 and Z55, Volume 28, The Omega Engineering Handbook, Vol. 28.
In order to render such devices more accurate, laser beam sighting devices have been used to target the precise center of the energy zone. See, for example, Pages C1-10 through C1-12 of The Omega Temperature Handbook, Vol. 27. Various sighting devices such as scopes with cross hairs have also been used to more accurately identify the center of the energy zone to be measured. See, for example, Pages C1-10 through C1-21 of The Omega Temperature Handbook, Vol. 27.
The use of a laser to pinpoint the center of the energy zone does not, however, provide the user with an accurate definition of the actual energy zone from which the measurement is being taken. This inability frequently results in inaccurate readings. For example, in cases where the area from which radiation limits is smaller than the target diameter limitation (too far from or too small a target), inaccurate readings will occur.
One method used to determine the distance to the target is to employ an infrared distance detector or a Doppler effect distance detector or a split image detector similar to that used in photography. However, the exact size of the energy zone must still be known if one is to have any degree of certainty as to the actual area of the surface being measured. This is particularly true if the energy zone is too small or the surface on which the energy zone encompasses is irregular in shape. In the case where the surface does not fill the entire energy zone area, the readings will be low and, thus, in error.
Similarly, if the surface is irregularly shaped, the readings will also be in error since part of the object would be missing from the actual energy zone being measured. Thus, the use of a single laser beam to the apparent center of the energy zone does not insure complete accuracy since the user of the radiometer does not know specifically the boundaries of the energy zone being measured.
As will be appreciated, none of the prior art recognizes this inherent problem let alone offers a solution to the problems created thereby.